1. Field of the Invention
The present invention relates generally to submerged impellers and, more particularly, to impellers used in generating metal foam.
2. Description of the Prior Art
There is a considerable demand for materials having high strength and low weight characteristics for use in manufacturing various articles. Such materials are very much in demand in the automobile and construction industries. To meet this demand, metal foam has been proposed. Metal foam is generally formed by introducing a gas into a molten metal bath to generate a foam on the surface thereof. Due to its high strength to weight ratio, aluminum is a favoured metal to use in generating a foam, although other metals can also be used. The foam is then removed and formed or cast into the desired shapes. Various methods have been proposed for introducing the gas into the molten metal bath. Such methods include the use of gas generating additives, blowing of air etc. With regard to the latter method, various apparatus and systems are known for blowing a gas into the molten metal. Such apparatus include nozzles, impellers and other such devices.
In U.S. Pat. No. 5,334,236, there is described a metal foam generating system wherein air is introduced by means of a gas nozzle at the end of a supply tube or a hollow rotating impeller having a plurality of openings through which the gas is passed. In both cases, the tube or impeller is mounted on an angle into the metal bath through an opening. There is no teaching in this patent as to how such opening is sealed to prevent the molten metal from leaking. Further, the shafts used in forming the tubes or impellers are formed from stainless steel due to the fact that they are immersed in molten metal. Nevertheless, such shafts are known to become deteriorated after prolonged immersion in the molten metal and must be replaced often. Another deficiency in these known gas introduction systems is that since the shafts are provided in an angled manner into the molten metal bath, the length of the shafts must be adjusted if the depth of the bath is increased. Apart from the drive mechanism requirements of such an arrangement, it will be understood that the cost for each shaft would also be greater. This, compounded with the need for constant replacement of the shafts, results in a light cost of operation.
In U.S. application Ser. No. 60/312,757, sharing a common inventor with the present application, an improved metal foam generating and casting system is provided. In this system, a metal foam is generated by introducing a gas into the bottom of the metal bath to generate bubbles. The bubbles are then allowed to rise through a riser tube connected to a die cavity. The bubbles then form a foam inside the cavity. After the cavity is filled, it is allowed to cool and the formed metal foam article is retrieved. In this case, the generation of bubbles at a specific location is desired. This reference provides a porous nozzle located at the bottom of the molten metal bath, positioned generally directly under the riser tube. Although such porous nozzle results in the desired foam generation, a rotating nozzle is believed to improve the foam characteristics. However, the rotating nozzle shafts known in the art have various disadvantages as described above. In this specific application, one other disadvantage is that, with angled impeller shafts, it is often not possible to ensure that the formed bubbles are introduced into the riser tube. Further, the above mentioned system involves the pressurization of the foaming chamber. In such case an adequate seal around the impeller is needed in order to prevent leakage. Such seal is difficult to establish in situations where the impeller is introduced through the side of the molten metal bath.
Thus, there exists a need for an improved impeller system for generating metal foam.